This invention relates to an optical power limiting device for the protection of thermal imaging apparatus and the like.
CO.sub.2 -lasers with medium (currently common) power intensities are capable of blinding or destroying IR-detectors, such as those utilized for example, in heat image devices. Conventional filters either provide no protection against such high intensity radiation or do so only with a considerable loss of sensitivity, because the operating frequencies of the CO.sub.2 -laser are within the maximum of the ambient radiation.
One countermeasure may be the use of very narrow-band selective filters which effectively exclude the laser radiation without impairing the sensitivity of the heat image device. This method is practical as long as the threat is limited to a few discrete frequencies, but would fail in the case of variable lasers, whose frequency can be changed within certain limits.
A widely used solution to this problem consists of a filter or a screen with an externally controlled intensity-dependent transmissibility, thus providing an optical power limiting device. However, arrangements which use an external control mechanism are unsuitable, at least as a primary protection, because of the long idle times.
German Patent Document DE 36 05 635 A1 discloses a power limiting device for laser light without an outside control mechanism, in which the transmissibility decreases considerably with rising intensity of the radiation to be limited. It comprises a breakthrough path consisting of a gaseous, liquid or solid material. Power limitation is based on the physical principle that, with rising intensity of the incident radiation, a plasma is created by means of multi-phonon absorption and cascade ionization which protects from the laser radiation. A disadvantage of this arrangement, however, is that the equipment expenditures are relatively high. Thus, corresponding receptacles are required for the breakthrough path, for example, in the case of liquid or gaseous materials. Since the breakthrough threshold value depends on the pressure inside the materials, a constant controlling of pressure is also required, and unintentional pressure loss may result in malfunctioning.
European Patent Document EP 21 336 A1 describes an optical image apparatus comprising a prism, with incident radiation being guided into the prism and onto its base. On the base, there is a layer whose dielectric constant is negative in the relevant wave length range. (The dielectric constant is a measure of the extent to which a material transmits an incident light beam, as well as to extent to which it absorbs incident light.) A medium with a variable refraction index is arranged in contact with this first layer. The refraction index of the medium, and thus the degree of reflection on the base on the prism, can be changed by various control mechanisms, such as for example, an electrochemical, a photochromic or an electrochromic process.
In Applied Optics, Vol. 13, No. 1, Jan. 74, Pages 17 to 19, a beam reducer is described, in which a layer is arranged between two prisms. The refraction index of the layer is smaller than that of the prism material. By changing the layer thickness, the transmission ratio of the arrangement can be changed.
U.S. Pat. No. 3,834,793 describes a dichromatic mirror made of a substrate having a reflectivity that is as high as possible, to which several layers are applied. At least one of such layers is VO.sub.2. Incident white light passes through the individual layers and is partially reflected on the boundary surfaces, and completely reflected on the substrate. Due to interference of the light reflected on the different boundary surfaces, certain wavelengths are amplified, and others are extinguished. By the heating of the VO.sub.2 -layer and the resulting change of the refraction index, the color of the reflected light can be changed.
U.S. Pat. No. 4,795,240, discloses an infrared shutter which has a thin layer of a thermochromic material, the transmissibility of which decreases considerably as the temperature rises. The transmissibility of the layer can be controlled by an outside control mechanism. For this purpose, an electric current of a defined intensity is conducted through a layer which is adjacent to the thermochromic layer. The resulting resistance heat is used to control the temperature of the thermochromic layer.
U.S. Pat. No. 4,615,587 discloses a thermooptic thin-film modulator by means of which image information can be impressed into a thin thermooptic layer whose transmissibility falls considerably as the temperature rises, and can be deleted again. For this purpose, rapid temperature control of the thermooptic layer is required, which is achieved by conducting an electric current through a metal layer adjacent to the thermooptic layer for the purpose of resistance heating, and at the same time, using a cooling liquid to dissipate a portion of the generated heat. By interruption of the current, the equilibrium of the heat production and heat loss can be altered and, as a result, the temperature of the thermooptic layer can be changed very rapidly.
It is an object of the present invention to provide an optical power limiting device for a radiation which permits secure operation with low equipment-related expenditures.
This object is achieved by the power limiting device according to the invention, which has a transmissibility that is a function of the intensity of the incident radiation within a wave length range of from 1 to 20 .mu.m. It comprises a prism with at least one plane boundary surface to which two layers, arranged above one another, are applied. One layer (active layer) consists of a material whose real component of the dielectric function is negative in the relevant frequency range; the other (a dispersion layer) consists of a material whose refraction index is closely dependent on the temperature in the frequency interval of interest. The radiation to be limited is guided into the prism and onto the plane boundary surface comprising the above mentioned layers. A prism, in this case, is generally a body made of a light-transmitting and refracting material which is bounded by at least two intersecting planes. The refraction index of the prism material must be greater than that of the surrounding medium, which is generally air.
The invention is based on utilization of the resonance characteristics of surface plasmons which propagate on the active layer. Changing the plasmon resonance characteristics causes a change in reflectivity during reflection of incident radiation on the plane boundary surface of the prism. This change of the reflectivity is utilized to limit the power that is transmitted.
The resonance characteristics of the surface plasmons are significantly influenced by the optical characteristics of the adjacent dispersion layer, particularly by its refraction index. Thus, by changing the refraction index of the dispersion layer, a change of the reflectivity can be achieved. In the case of the present invention, this change of the refraction index is caused by heating. That is, the dispersion layer consists of a material whose refraction index is highly dependent on its temperature (thermorefractive material).
It is crucial that the required heating in this case is controlled directly by the radiation to be limited, and not by an outside control mechanism. That is, the incidence of high intensity radiation heats the arrangement due to residual absorption of the participating materials.
In the case of low light intensities, the light which is incident in the prism is reflected almost completely. In the case of high intensities, the absorption starts by means of the generating of surface plasmons, and the reflected light intensity is reduced very considerably.
The dispersion layer is microscopically thin--in the nm/.mu.m-range--so that its heating and the desired change of the reflectivity can take place very rapidly.